On morphological and developmental grounds, we postulated that the mammalian Peyer's patch-type intestinal lymphoepithelial tissues might represent the central lymphoid organs for differentiation of the population of lymphoid cells responsible for humoral immunity. A method for neonatal surgical excision of appendix, sacculus rotundus, and Peyer's patches of rabbits has been described. These procedures can be carried out with a mortality rate of 20 to 30 percent in rabbits 1 to 2 weeks of age. Such manipulation leads to a specific antibody deficiency syndrome in which cell-mediated immune reactions remain intact. It is concluded that lymphoepithelial Peyer's patch-type intestinal tissues of mammals represent a homologue of the avian bursa of Fabricius.